There is well-known heat-development in which developing is carried out by applying heating to obtain a black and white, or color image. A heat-developable photosensitive material for use in diffusion transfer process is also known, in which an image formed through heat-development is transferred to a iamge-receiving layer from a photosensitive material.
A heat-developable photosensitive material conventionally comprises a support having thereon a binder, photosensitive silver halide emulsion, reducing agent, and optionally a dye-providing material, organic silver salt or various photographic additives. The diffusion transfer heat-developable photosensitive material includes one in which the photosensitive material has a layer capable of receiving silver or dye and another one in which, apart from the photosensitive material, an image-receiving material having a layer capable of receiving silver or dye is used therewith.
Among these heat-developable photosensitive materials, there is used a dye providing material capable of forming or releasing a dye upon heat-development to obtain a color image. In such a color heat-developable photosensitive material, from the viewpoint of image sharpness and storage stability of color image, there is preferably employed a system in which a diffusible dye is formed or released and the dye is duffusion-transferred to an image-receiving material.
Such dye-providing materials used in the diffusion transfer type heat-development photosensitive material have been known in the art, most of which are those for use in wet diffusion transfer process (so-called instant photography). These dye-providing materials are classified into two types; one type is one capable of releasing a diffusible dye corresponding to development of silver halide and another type is one capable of releasing a diffusible dye corresponding reversedly to the development of silver halide.
One of the latter type is a dye-providing material capable of releasing a dye upon reaction with a silver ion as a component of silver halide or organic silver salt which is remained undeveloped, or a water-soluble silver ion complex thereof.
Embodiments in which this type of the dye-providing material is applied to wet diffusion transfer process are disclosed in U.S. Pat. Nos. 4,362,806, 3,719,489 and 4,375,507. An embodiment in which the dye-providing material is applied to diffusion transfer type heat-development is disclosed in JP-A 59-180548(1984).
One disadvantage of these dye-providing materails is that it is necessary to develop at a high temperature taking a long time to obtain a sufficiently high density. In other words, it is impossible to obtain a sufficiently high density at a lower temperature over a shorter period of time.
Another disadvantage is that, when the heat-developable photosensitive material containing the dye-providing material is aged, the dye-providing material reacts with a silver ion during storage to release a diffusible dye. Thus, the heat-developable photosensitive material is susceptible to be fogged on storage.